Sema: Lift obsolete restriction on protocol extensions via typealias

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -1347,9 +1347,6 @@ ERROR(extension_constrained_inheritance,none,
       "inheritance clause", (Type))
 ERROR(extension_protocol_inheritance,none,
       "extension of protocol %0 cannot have an inheritance clause", (Type))
-ERROR(extension_protocol_via_typealias,none,
-      "protocol %0 in the module being compiled cannot be extended via a "
-      "type alias", (Type))
 ERROR(objc_generic_extension_using_type_parameter,none,
       "extension of a generic Objective-C class cannot access the class's "
       "generic parameters at runtime", ())

lib/Sema/TypeCheckDecl.cpp

@@ -7808,18 +7808,6 @@ void TypeChecker::validateExtension(ExtensionDecl *ext) {
   // FIXME: Probably the above comes up elsewhere, perhaps getAs<>()
   // should be fixed.
   if (auto proto = extendedType->getCanonicalType()->getAs<ProtocolType>()) {
-    if (!isa<ProtocolType>(extendedType.getPointer()) &&
-        proto->getDecl()->getParentModule() == ext->getParentModule()) {
-      // Protocols in the same module cannot be extended via a typealias;
-      // we could end up being unable to resolve the generic signature.
-      diagnose(ext->getLoc(), diag::extension_protocol_via_typealias, proto)
-        .fixItReplace(ext->getExtendedTypeLoc().getSourceRange(),
-                      proto->getDecl()->getName().str());
-      ext->setInvalid();
-      ext->getExtendedTypeLoc().setInvalidType(Context);
-      return;
-    }
-
     GenericEnvironment *env;
     std::tie(env, extendedType) =
         checkExtensionGenericParams(*this, ext, proto, ext->getGenericParams());

test/decl/ext/protocol.swift

@@ -921,9 +921,10 @@ extension BadProto1 {
   }
 }
 
+// rdar://problem/20756244
 protocol BadProto3 { }
 typealias BadProto4 = BadProto3
-extension BadProto4 { } // expected-error{{protocol 'BadProto3' in the module being compiled cannot be extended via a type alias}}{{11-20=BadProto3}}
+extension BadProto4 { } // okay
 
 typealias RawRepresentableAlias = RawRepresentable
 extension RawRepresentableAlias { } // okay
@@ -948,6 +949,6 @@ class BadClass5 : BadProto5 {} // expected-error{{type 'BadClass5' does not conf
 typealias A = BadProto1
 typealias B = BadProto1
 
-extension A & B { // expected-error{{protocol 'BadProto1' in the module being compiled cannot be extended via a type alias}}
+extension A & B { // okay
 
 }